For the purpose of reducing the cost of optical networks in recent years, optical repeaters that do not utilize photoelectric conversion have been used, and the repeating distance thereof has been increased. In order to extend a repeating segment based on a non-reproducing method (i.e., a method not utilizing photoelectric conversion) by increasing the repeating distance, it is necessary to focus attention on the transmission quality of signal light to be transmitted. For example, with regard to an optical amplifier, for performing optical repeating, provided on an optical transmission path, polarization hole burning (PHB) is known to occur in accordance with the wavelength of the signal light, as discussed in “Spectral Dependence of Polarization Hole-Burning”, Davidson C. R. et al., Optical Fiber Communication Conference, 2006 and the 2006 National Fiber Optic Engineers Conference (OFC 2006) 5-10 Mar. 2006 and “Polarization Dependent Gain in Erbium-Doped Fiber Amplifiers: Computer Model and Approximate Formulas”, Paul Wysocki and Vincent Mazurczyk, Journal of Lightwave Technology, Vol. 14, No. 4, April 1996. Transmission quality may be adversely affected depending on the wavelength of the signal light to be amplified. As discussed in “Erbium-Doped Optical-Fiber Amplifier”, Shoichi Sudo, Optronics Co., Ltd., pp. 59-61 with regard to, for example, a rare-earth-doped optical-fiber amplifier, PHB is a phenomenon in which the gain of an optical amplifier fluctuates depending on the polarization state of signal light and excitation light.
When signal light with a high power level is input to a rare-earth-doped optical-fiber amplifier, the gain of light having a polarization direction that is parallel to the polarization direction of the input signal light is reduced in a PHB-occurring wavelength range. The fluctuating gain in the rare-earth-doped optical-fiber amplifier has an effect not only on the signal light but also on amplified spontaneous emission (ASE) occurring within the optical fiber. ASE is light in a non-polarized state and includes a polarized component that is parallel and a polarized component that is orthogonal to the polarization direction of the signal light. In the ASE, the polarized component parallel to the signal light is solely affected by the fluctuating gain caused by PHB. Specifically, the occurrence of PHB causes the gain of the signal light and the gain of the polarized component, of the ASE, parallel to the signal light to be reduced but does not cause the gain of the polarized component, of the ASE, orthogonal to the signal light to be reduced. Therefore, in the ASE occurring in the optical fiber, a difference occurs between the gain of the polarized component parallel to the signal light and the gain of the polarized component orthogonal to the signal light. Consequently, a proportion of the polarized component, of the ASE, orthogonal to the signal light relatively increases as compared with when there is no PHB, thus affecting the transmission quality of the signal light within the corresponding wavelength range, particularly, an optical signal-to-noise ratio (OSNR).
In view of the characteristics of the aforementioned optical amplifier, there is still room for improvement with regard to the transmission quality of signal light in an optical transmission system that performs optical repeating.